SorenH · Jan 29, 2023

RR P38 1998, 4.6 V8 HSE petrol

When depressing the brake pedal, the dashboard, rev, etc, goes out. However, it sometimes also does it without pressing the brake pedal.

I have removed the black brake light switch (with three connector pins), and the revs drop down or are very erratic.

At the same time, there are a lot of other faults messages shown. Not sure if that is related to the above issue, as I think that there is something that discharges the battery.

Thanks

Søren

kermit_rr · Jan 29, 2023

Huge guess without much to go on, i wonder if your fuse box is ok?

Ratae · Jan 29, 2023

What happens when you fit a known good fully charged battery?
Have you fully charged your existing battery with a battery charger?

Datatek · Jan 30, 2023

I think on the petrol, there is a vacuum pipe for cruise control associated with the brake switch, if that is leaking, could it affect the revs and/or idling?
^^^^^^Battery voltage is a good call.

pwood999 · Jan 30, 2023

+1 on check battery first. Also check the ground connection behind right footwell panel.

kermit_rr · Jan 30, 2023

Datatek said:
I think on the petrol, there is a vacuum pipe for cruise control associated with the brake switch, if that is leaking, could it affect the revs and/or idling?
^^^^^^Battery voltage is a good call.

The vacuum is from a pump, not from the manifold, so any leaks means it simply stops working, engine speed is unaffected

Datatek · Jan 30, 2023

kermit_rr said:
The vacuum is from a pump, not from the manifold, so any leaks means it simply stops working, engine speed is unaffected

OK thanks.

Grrrrrr · Feb 3, 2023

SorenH said:
RR P38 1998, 4.6 V8 HSE petrol

When depressing the brake pedal, the dashboard, rev, etc, goes out. However, it sometimes also does it without pressing the brake pedal.

I have removed the black brake light switch (with three connector pins), and the revs drop down or are very erratic.

At the same time, there are a lot of other faults messages shown. Not sure if that is related to the above issue, as I think that there is something that discharges the battery.

Thanks

Søren

Instrument cluster comes out easily enough, although be real careful with the screws as the plastic breaks for fun. There's only one connector but might be worth a bit of electrical contact cleaner in the connectors.

That's assuming the revs drop on the dial but the engine still sounds fine. If the engine is really dying I'd be checking the earth straps around the engine bay and the battery terminal connectors are tight. I made a spacer from some copper pipe for someone's battery terminal clamps.

SorenH

New Member

kermit_rr

Very

Ratae

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Datatek

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pwood999

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kermit_rr

Very

Datatek

Well-Known Member

Grrrrrr

Technician, Bodgit & Scarper Ltd

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